Nitrogen-containing ceramic precursors

A number of processible nitrogen-containing ceramic precursors are also discussed. These include polyalazanes (Jensen, Chapter 32), cross-linkable vinyl substituted polysilazanes (Schwark, Chapter 5), mixed sol-gels fi om aminolysis reactions (Gonsalves, Chapter 16), and polyborazines (Sneddon and Paine, Chapter 27 and Kimura, Chapter 28) which serve as precursors to AIN, Si3N4, BN/AJN composites, and BN, respectively. Arylene and alkylene bridged polysilsesquioxanes (Loy, Chapter 11) and carborane-polysiloxanes (Keller, Chapter 31) have bIso been employed to make modified silicas. 

It has become clear that the carbon-rich hb-PYs are readily curable (from ca. 150 °C), thermally stable (up to ca. 550 °C), and pyrolytically carboniz-able (yield up to 80% at 900 °C). Furthermore, their triple bonds are easily metallizable by the complexations with cobalt carbonyls. Since the polymer complexes contain a large number of metal atoms, we tried to utilize them as precursors for fabrication of metalloceramics. The pyrolyses of the polyyne-cobalt complexes at 1000 °C for 1 h under nitrogen furnished ceramic products 83 and 84 in 50%-65% yields (cf., Scheme 31). All the ceramics were magnetizable and could be readily attracted to a bar magnet. 

The use of large, nitrogen-centered substituents at a silanetriol center does allow the isolation and characterization of stable sUanetriols as shown in equation (24). These compounds can be used to prepare transition metal sUoxide complexes (Scheme 34), still containing potentially moisture-sensitive Si N bonds that may be precursors to metal-containing ceramics for reviews of these compounds, see References 193-195. 

X HE USE OF CHEMICAL APPROACHES to improve the processing, properties, and performance of advanced ceramic materials is a rapidly growing area of research and development. One approach involves the preparation of organometallic polymer precursors and their controlled pyrolysis to ceramic materials. This chapter will review the preparation and application of silicon-, carbon-, and nitrogen-containing polymer systems. However, the discussion is not exhaustive the focus is on systems with historical significance or that demonstrate key technological advances. 

CP/MAS (with natural abundance or N-enriched samples), among others Studies of nitrogen/carbon-containing ceramics derived from pyrolysis of organic polymer precursors and related materials Measurements involving molecules adsorbed on graphite... 

These examples illustrate that augmenting the thermal crosslinking reaction of vinyl-substituted precursors with platinum catalysts provides a means to obtain crosslinked precursors more rapidly or with enhanced ceramic yield. Still, the processing times described for both the thermal and the platinum-catalyzed cure methods are too long for rapid ceramic processing. In addition, hydrosilylation of nitrogen-containing silicon compounds is difficult at best. 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

High temperature stability of these nonoxide fibers in air is another critical problem. Thermal stability of ceramic fibers derived from polymeric precursors is of special concern mainly because, as mentioned above, they frequently have undesirable phases present in them. Polycarbosilane-derived SiC fibers, such as Nicalon or Tyranno, involve a thermal oxidation curing process as described above and can contain as much as 10 wt % oxygen (Okamura and Seguchi, 1992). Such fibers decompose at temperatures above 1200 C in a nitrogen or argon atmosphere with SiO and CO gas evolution ... 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

Metallic nanopartides were deposited on ceramic and polymeric partides using ultrasound radiation. A few papers report also on the deposition of nanomaterials produced sonochemically on flat surfaces. Our attention will be devoted to spheres. In a typical reaction, commerdally available spheres of ceramic materials or polymers were introduced into a sonication bath and sonicated with the precursor of the metallic nanopartides. In the first report Ramesh et al. [43] employed the Sto-ber method [44] for the preparation of 250 nm silica spheres. These spheres were introduced into a sonication bath containing a decalin solution of Ni(CO)4. The as-deposited amorphous clusters transform to polyciystalline, nanophasic, fee nickel on heating in an inert atmosphere of argon at a temperature of 400 °C. Nitrogen adsorption measurements showed that the amorphous nickel with a high surface area undergoes a loss in surface area on crystallization. 

Sauter (2000) [266] investigated the structure of amorphous boron carbonitride of composition B31C37N32 by X-ray and neutron diffraction as well as by NMR spectroscopy. It was shown that annealed precursor-derived B-C-N ceramics (1200 K < T < 1600 K) contain predominantly tricoordinated boron, carbon and nitrogen atoms arranged in hexagonal rings, or fragments of them, as structural units. No phase separation occurred. 

Polyvinylidene chloride (PVDC) and polyvinyl alcohol (PVA) microspheres were carbonized on ceramic membrane to fabricate activated carbon membrane for coke furnace wastewater treatment [15]. A ceramic tube was dipped into a polymer latex containing 70 wt% PVDC and PVA microspheres of 0.10-0.15 pm to form aggregates of polymeric microspheres on and within (within pores of) the ceramic pipe. The precinsor was heated at 300°C and further to 750°C for carbonization. Major decomposition of the polymeric precursor seems to occur at 300°C. By nitrogen adsorption applying Horvath and Kowazoe method the membrane was found to have micropores of 0.7-0.8 run in diameter and meso-pores of 2-20 nm. Hence, the membrane has bimodal pore size distribution. The molecular weight cutoff of the membrane was ca. 10,000 Dalton. 

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